Advanced NOx Control With Selective Reduction Agents

The effectiveness of combustion modifications for the control of nitrogen oxide emissions from coal fired combustors is most often limited by problems due to carbon burnout or flame impingement. This paper presents new data on the use of selective reducing agents suggesting that a hybrid control scheme is possible which uses combustion modification to provide those conditions which optimize the selective reduction process. Very low emission levels appear possible that can presently only be achieved by catalytic reduction. The experimental studies were conducted in a tunnel furnace which simulated the thermal environment within a pulverized coal boiler. Application of each of the agents (ammonia, urea, cyanuric acid, and ammonium sulfate) to an overall fuel lean environment produced NO reduction behavior very similar to that of thermal deNOx. However, if the agent was added to the fuel rich zone of a rich/lean staged combustor, very high NO reductions were obtained after the leanout point. The result of the staging was to extend the effectiveness of the agent to lower temperatures relative to overall lean injection. Parametric variations indicated that, in addition to temperature, the most important variable was the rich zone stoichiometry. Kinetic modeling suggests that the rich zone acts primarily as a source of CO. At the rich/lean transition the CO is oxidized and excess OH is produced by the usual chain branching reactions. For low initial CO concentrations the excess radicals are consumed by: NH3 + OH = NH2 + H2 0 (50) HNCO + H = NH2 + CO (92) The NH2 is then available for reaction with NO to eventually yield N2. The strong rich zone stoichiometry dependence is exerted mainly through the amount of CO supplied to the lean zone. Insufficient CO will limit the extent of the initial NH3 or HNCO reaction.